Variable coaxial attenuator utilizing movable lossy dielectric block



J. E. MCFARLAND VARIABLE COAXIAL ATTENUATOR UTILIZING MOVABLE LOSSY DIELECTRIC BLOCK 10, 1962 2 Sheets-Sheet 1 FIG. 2

FIG.I

INVENTOR. JAM ES E. McFARLAND ATTORNEY March 7, 1967 Original Filed Dec.

" IIIIIII 13 Q Xu X #4 L EfiU W M V I. W A 4 TI I I I I I F I I I I I I I I I I m 22 M J. E- M FARLAND VARIABLE COAXIAL ATTENUATOR UTILIZING MOVABLE LOSSY DIELECTRIC BLOCK March 7, 1967 2 Sheets-Sheet 2 Original Filed Dec. 10,

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INVENTOR JAMES E. McFARLAND BA ,0 )M (4 v/1 AT TORN EY United States PatentOfifice 3,398,461 Patented Mar. 7, 1967 t 1 3,308,401 j VARIABLE COAXIAL ATTENUATUR UTILIZING MOVABLE LOSSY DHELECTRIC BLOCK James E. McFarland, Huntington, N.Y., a'ssignor to The Narda Microwave Corporation, Plainview, N.Y., a corporation of New York Continuation of application Ser. No. 243,423, Dec. 10, 1962. This application Oct. 21, 1965, Ser. No. 506,437 4 (Zlaims. (Cl. 333-81) This application is a continuation of application Serial No. 243,423, filed December 10, 1962, now abandoned.

This invention relates to improvements in microwave attenuator devices, and more particularly to adjustable direct reading attenuators for use in coaxial line systems operating at microwave frequencies, for example of the order of 9 kilomegacycles per second.

The principal object of the invention is to provide an adjustable attenuator that can be inserted between two coaxial line sections in a microwave system without introducing troublesome impedance mismatch, or variations in impedance with variations in the degree of attenuation.

Another object is to provide an adjustable coaxial line attenuator wherein variations of phase shift with variations of attenuation are minimized.

A further object is to provide an attenuator of the foregoing type that can be calibrated accurately for direct reading, wherein the attenuation is substantially independent of frequency throughout a relatively wide frequency band, enabling the use of a simple single calibration without need of correction charts or tables.

Another important object is to provide a device fulfilling the above objects that is simple in design and construction, trouble-free, and dependable in operation.

The foregoing objects are met by a microwave transmission line section comprising an inner conductor connected to the inner conductors of the input and output coaxial lines and disposed midway between two ground plane members, each having a low-loss conductive region and a high-loss dissipative region. The ground plane memhere are mechanically and electrically joined to form a unitary slide structure which is mounted for translatory motion to expose a controllable amount of the length of the inner conductor to the lossy environment.

The invention will be described with reference to the accompanying drawings, wherein:

FIGURE 1 is a plan view of a presently preferred embodiment,

FIGURE 2 is an end elevation of FIGURE 1,

FIGURE 3 is a sectional view, taken in the plane 33 of FIGURE 1,

FIGURE 4 is a section taken in the plane 44- of FIG- URE 3,

FIGURE 5 is a section taken in the plane 5-5 of FIG- URE 3, and

FIGURE 6 is a section taken in the plane 6-6 of FIGURE 3.

Referring to FIGURES 1 and 2, theexternal structure of the attenuator is a generally rectangular housing 1 with a window 2 in its upper face for viewing a calibrated scale 7 mounted rotatably within the instrument. The window 2 is provided with a fiducial mark 8. Fittings 9 and 10 disposed at the sides of the housing 1 are adapted for connection to mating fittings at the adjacent ends of input and output coaxial lines, not shown. A manually rotatable knob 11 at the front face of the housing 1 is provided scale 7 to indicate said amount.

' trally of the housing 1 from one side wall to. the other.

The assembly of conductors 13, 14 and is supported on dielectric beads 13' and 14 set in correspondingapertures formed in the side walls.

As shown in FIGURE 3, the inner conductor 15 lies midway between, and parallel to the facing'parallel surfaces of a pair of block members 16 and 17 forming part of a slide assembly to be described.

The upper block member 16 appears in plan view in FIGURE 4. It consists of a forward part 18 made of aluminum or similar electrically conductive material, and a rearward part 19 made of a composition of finely divided iron particles suspended in a dielectric binder, or similar material exhibiting substantial dissipative property at microwave frequencies. The boundary 20 between the parts 18 and 19 is in the form of a wide shallow V, with its open end facing the lossy part 19, as shown in FIG- URE 4. The parts 18 and 19 are cemented together at the boundary 20 as by a thermosetting epoxy resin, to form the unitary block member 16. The lower block member 17 (FIGURE 3) is similar to the upper block member, consisting of a conductive part 21 and lossy part 22 joined together at boundary 23 that is shaped like the boundary 20. I

The block members 16'and 17 are secured near their rear edges to a spacer 24 as shown in FIGURES 3 and 4. The forward edge of the upper block 16 is formed with downwardly extending rectangular ridges 25 and 26 for spacing the forward edges of the blocks 16 and 17 by the same distance as the rear edges are spaced by the spacer 24, as shown in FIGURES 4 and 6. The forward edges of the block members 16 and 17 are recessed, as shown best in FIGURES 4 and 6, to receive a rectangular plate 27 which is secured to both blocks and is provided with a central aperture threaded to cooperate with a leadscrew 28.

The blocks 16 and 17 with the spacer means and the plate 27 form a unitary slide assembly adapted to move forwardly and rearwardly within the housing to expose less or more, respectively, of the length of the inner conductor 15 to the lossy environment of the dissipative parts 19 and 22. For this purpose, the inner surface of the housing 1 is formed near its upper wall with shoulders 29 and 36 extending from the front to the rear (see FIG- URES 4 and 5) and acting as guides or Ways in sliding contact with the left and right upper edges of the slide assembly. The leadscrew 28 is provided with a shaft 31 (FIGURE 3) mounted in the front wall of the housing 1 for rotation by the knob 11. Translational motion of the shaft 31 is prevented by a collar 32 and washers 33, 34 and 35. j

The dial 7 is a circular disc secured to a central shaft 36 (FIGURE 3) rotatably supported in a bearing 37 secured in an aperture in the top wall of the housing by a bearing retainer 38. The shaft 36 extends below the dial 7 to accommodate a dial cord 39 passing through a diametral hole in the shaft, and wrapped around the shaft as shown in FIGURES 3 and 4. One end of the dial cord 39 is secured to the forward end'of the slide assembly by a screw 40 threaded in the plate 27. The other end of the dial cord 39 is attached to a tension spring 41 secured to the rear face of the lower block member 17. A guide member 42 having a slotted head is disposed on the upper face near the rear edge of the block member 16 to guide the dial cord 39 between the spring 41 and the shaft 36.

The foregoing arrangement rotates the dial 7 in accordance with the transl-atory motion of the slide assembly, to indicate the position of the slide assembly with respect to the inner conductor 15. Rotation of the screw 40 enables set-up adjustment of the positional relationship between the dial and the slide assembly, by increasing or decreasing the effective length of the dial cord 39 between the shaft 36 and the screw 40.

The spacing between the block members 16 and 17 is so related to the diameter of the inner conductor that the characteristic impedance of the structure acting as a transmission line substantial-1y matches that of the coaxial lines between which the attenuator is to be connected. The dimensional relationship required in any particular instance may be calculated or determined experimentally. In a presently preferred design for operation with SO-ohm lines, the diameter of the inner conductor is 0.156 inch and the spacing between the block members is 0.285 inch.

The angle formed by the V-shaped boundary between the conductive and lossy portions of the blocks 16 and 17 is a design consideration depending upon the desired range of attenuation adjustment and the positional range of the slide assembly. In a typical attenuator proportioned substantially as shown in the drawing and designed to adjust from zero to 22 db, the apex angle of the V is 144 /2 degrees.

In the operation of the described attenuator, the inner surfaces of the block members 16 and 17 cooperate with the inner conductor 15 to act as a transmission line having characteristics substantially the same as those of a coaxial line, the blocks serving as a grounded outer conductor. When the slide assembly is in its extreme rearward position, the center conductor 15 is exposed substantially' solely to the highly conductive surfaces of the block portions 18 and 21, and the attenuation introduced by the device is simplythat of a short low-loss line section, essentially zero. As the slide assembly is moved toward its forward position, an increasing portion of the length of the inner conductor 15 is exposed to the dissipative portions 19 and 22, and the attenuation is correspondingly increased.

The leadscrew 28 permits precise adjustment of the attenuation, and the dial 7 may be permanently calibrated to indicate the adjustment in terms of db. The attenuation is substantially independent of frequency over a wide range, for example from 7 krnc to ll kmc. ina typical design. Thus the dial readings may be used directly, without correction charts or tables.

Since the ground planes presented to the inner conductor remain at a constant distance from it and cover a constant length, varying only in the ratio of low-loss to dissipative surface exposed to the inner conductor, varia tions in characteristic impedance and phase shift with adjustment of attenuation are minimized.

It will be apparent from the foregoing that the several objects of the invention are fulfilled by the described embodiment thereof. Although a particular preferred arrangement is disclosed in detail, it will be understood that many variations are possible within the spirit of the invention. For example, the vibrating actuator could be of a known pneumatically operated type, energized from the propellant gas supply. The liquid control valve may be, for certain applications, constructed for example like a poppet valve or other arrangement adapted to be operated in a vibratory fashion.

I claim:

1. An adjustable microwave attenuator comprising an elongated linear conductor, a slide member presenting a fiat ground plane surface adjacent and parallel to said conductor, said slide member com-prising, in the ground plane,

, a low-loss conductive region and a high-loss dissipative region comprising a lossy dielectric block affording series loss paths parallel to the ground plane and shunt loss paths perpendicular to the ground plane and extending away from the conductor, and means supporting said slide member for motion transversely of said conductor and at a fixed distance therefrom to vary the ratio between the portions of the length of said conductor that are exposed to said low-loss and high-loss regions respectively.

2. An adjustable microwave attenuator for use in coaxial line systems, comprising an inner conductor, a slide member comprising a pair of rectangular block members disposed in spaced parallel relationship on opposite sides of said inner conductor to define parallel ground plane surfaces, each of said block members comprising, side by side in the ground plane, a low-loss conductive region and a high-loss dissipative region, the latter region comprising a lossy dielectric block affording series loss paths parallel to the ground plane surface and shunt loss paths extending away from the conductor perpendicular to the ground plane surface, means supporting said slide member for motion in a direction substantially perpendicular to the axis of said inner conductor and parallel to the facing surfaces of said block members, and means for adjusting the position of said slide member to vary the ratio between the portions of the length of said inner conductor that are exposed to said low-loss and high-loss regions respectively.

3. An adjustable microwave attenuator for use in coaxial line systems, comprising a conductive housing, an inner conductor extending across the interior of said housing, a slide member within said housing comprising a pair of block members disposed in spaced parallel relationship on opposite sides of said inner conductor to define parallel ground plane surfaces, each of said block members comprising, side by side with the ground plane surfaces, a low-loss conductive region and a high-loss dissipative region, the latter region comprising a lossy dielectric block affording series loss paths parallel to the ground plane surface and shunt loss paths extending away from the conductor perpendicular to the ground plane surface, the boundary between said regions as viewed from the mid-plane between the facing surfaces of said block members being skewed with respect to the axis of said inner conductor, and means supporting said slide member for motion transversely of the axis of said inner conductor and substantially parallel to the facing surfaces of said block members.

-4. An adjustable microwave attenuator for use in coaxial line systems, comprising a conductive housing, an inner conductor extending across the interior of said housing and insulated therefrom, the respective ends of said inner conductor being adapted to be connected to the inner conductors of input and output coaxial lines, a slide member within said housing comprising a pair of block members disposed in spaced parallel relationship on opposite sides of said inner conductor to define parallel ground plane surfaces, each of said block members comprising, side by side in the ground plane, a low-loss conductive region and a high-loss dissipative region, the latter region comprising a lossy dielectric block affording series loss paths parallel to the ground plane surface and shunt loss paths extending away from the conductor perpendicular to the ground plane surface, the boundary between said regions as viewed from the mid-plane between the facing surfaces of said block members being in the form of an obtuse angle, means supporting said slide member for rectilinear motion in the direction perpendicular to the axis of said inner conductor and substantially parallel to the bisector of said obtuse angle, means for ad justing the position of said slide member to vary the ratio between the portions of the length of said inner conductor that are exposed to said low-loss and high-loss 5 6 regions respectively, and means for indicating the position 2,837,720 6/1958 Saltzman et a1 33381 X of said slide member. 3,070,764 12/ 1962 Douglas 333-81 References Cited by the Examiner FOREIGN PATENTS UNITED STATES PATENTS 5 617,919 2/ 1949 Great Britain.

et aL 3334 X HERMAN KARL SAALBACH, Primary Examiner. 2,670,461 2/1954 Learned 33381 R. F. HUNT, Assistant Examiner. 

1. AN ADJUSTABLE MICROWAVE ATTENUATOR COMPRISING AN ELONGATED LINEAR CONDUCTOR, A SLIDE MEMBER PRESENTING A FLAT GROUND PLANE SURFACE ADJACENT AND PARALLEL TO SAID CONDUCTOR, SAID SLIDE MEMBER COMPRISING, IN THE GROUND PLANE, A LOW-LOSS CONDUCTIVE REGION AND A HIGH-LOSS DISSIPATIVE REGION COMPRISING A LOSSY DIELECTRIC BLOCK AFFORDING SERIES LOSS PATHS PARALLEL TO THE GROUND PLANE AND SHUNT LOSS PATHS PERPENDICULAR TO THE GROUND PLANE AND EXTENDING AWAY FROM THE CONDUCTOR, AND MEANS SUPPORTING SAID SLIDE MEMBER FOR MOTION TRANSVERSELY OF SAID CONDUCTOR AND AT A FIXED DISTANCE THEREFROM TO VARY THE RATIO BETWEEN THE PORTIONS OF THE LENGTH OF SAID CONDUCTOR THAT ARE EXPOSED TO SAID LOW-LOSS AND HIGH-LOSS REGIONS RESPECTIVELY. 